Abstract

The thickness of the eggshells of Blackbird Turdus merula, Song Thrush T. philomelos, Mistle Thrush T. viscivorus, and Ring Ouzel T. torquatus has previously been shown to have declined in Britain based upon measurements of eggshells in museum collections. A study of eggs of three of the same species collected in the Netherlands shows evidence of a decline in eggshell thickness of similar magnitude to that found in Britain for these species. This is the first time that eggshell thickness declines of such long duration have been found in two geographically separate areas. The cause of the decline is unknown, but began before the introduction of organochlorine pesticides.